This invention relates to improvements in a solid state image sensing device capable of reading invisible information recorded in the near-infrared region, and an image sensor adapted for reading object such as manuscripts or securities, etc. to obtain picture signals, and more particularly to an image sensor capable of reading invisible information recorded in the near-infrared region as well.
An example of a conventional typical image sensor will now be described with reference to FIG. 12. In this figure, a reflected light from an object 10 to be read such as manuscript, etc. is introduced through a lens 20 of the optical system and a filter 30 for cutting infrared rays, and light image is thus projected onto a solid state image sensing device 40. This solid state image sensing device 40 is a CCD, etc. for converting light image into picture signals. In addition, the infrared ray cut filter 30 serves to cut off rays of light (electromagnetic wave) of long wavelength to prevent lowering of the resolution resulting from the lens performance.
FIG. 13 shows the spectral sensitivity characteristic of the solid state image sensing device 40. In this figure, the abscissa indicates wavelength of incident light to the solid state image sensing device, and the ordinate indicates relative sensitivity when the maximum sensitivity is assumed to be reference value 1. The human eyes can sense electromagnetic wave of wavelength of approximately 380 nm (violet)-780 nm (red) to recognize it as visible light, but cannot recognize electromagnetic wave of wavelength except for this range (ultraviolet rays, infrared rays). However, the solid state image sensing device (CCD) has sensitivity also in the near-infrared region of 800-1000 nm which is invisible to the human being.
FIG. 14 shows transmission characteristics of color compensation filters which are one of the infrared ray cut filters. In this figure, the abscissa indicates wavelength of light, and the ordinate indicates transmissivity of light. CC-500 to CM-500 in this figure represent kind of filters. In the case where, e.g., the filter of CM-500 is used in the solid state image sensing device (CCD) shown in FIG. 12, incident light having wavelength of 600 nm is attenuated by 50%. Incident light of 600 nm is greatly attenuated, and most part of incident light of 700 nm is cut.
FIG. 15 shows an example of heat ray absorption filters which are different kind of the infrared ray cut filter. In this figure, the abscissa indicates wavelength of light and the ordinate indicates transmissivity of light. HA-20 to HA-60 in the figure represent kind of filters. In the case where, e.g., the filter of HA-20 is used in the solid state image sensing device (CCD) shown in FIG. 12, incident light having wavelength of 620 nm is attenuated by 50%. With respect to incident light of 800 nm, the transmissivity becomes equal to 10%. Namely, most part of the incident light of 800 nm is cut.
The image sensors are used for reading various papers (sheets) or the like. Among these papers, there are securities and/or certificates, etc. adapted so that, in order to give false prevention function or special information, when infrared rays are irradiated thereto, specific mark or information can be read out.
However, with the device using the infrared ray cut filter as described above, it is impossible to read marks or characters, etc. recorded in the near-infrared region.
FIG. 16 shows an example of trial manufacture of an image sensor adapted to have ability of reading characters, etc. recorded in the infrared region. In this figure, light obtained from object 10 to be read is branched into two rays of light by a half mirror 21. One ray is converged onto solid state image sensing device 40 provided with three CCD photosensitive pixel line sensors 42, 43, 44 corresponding to red, green and blue via lens 20 and infrared ray cut filter 30. In addition, the other light ray separated by the half mirror 21 is converged onto a solid state image sensing device 50 having a photosensitive pixel line sensor 45 for reading light in the infrared region as it is by a lens 22.
The solid state image sensing device 40 is adapted so that the three CCD line sensors 42, 43, 44 are formed on the substrate surface thereof. The first line sensor 42, the second line sensor 43 and the third line sensor 44 have configurations in which red filter, green filter and blue filter are respectively formed on the photosensitive pixel trains, and adapted for respectively reading the red component, the green component and the blue component.
The infrared ray cut filter 30 serves to cut infrared rays. Since employment of the infrared ray cut filter 30 results in improvement in convergence of light of the optical system, the solid state image sensing device 40 reads the object with a satisfactory resolution to output three color picture signals of R, G, B.
Moreover, in the solid state image sensing device 50, one line sensor is formed on the substrate. A filter (not shown) which allows light in the near-infrared region to be transmitted therethrough is formed on this line sensor to read marks or characters, etc. recorded in the near-infrared region. The result obtained by reading is outputted, e.g., as monochromatic picture signal of white and black.
However, with such configuration, the optical system for branching the optical path is complicated. As a result, the image sensor becomes large-sized and it is also difficult to carry out assembling with high accuracy. In addition, there is the problem that there results increased cost because plural solid state image sensing devices are required.
In order to solve such problem, an image sensor comprising both three color sensors and an infrared sensor on one chip is disclosed, e.g., in the Japanese Patent Application Laid-Open Publication No. 7-213126(1995). However, since there is employed the complicated structure in which the infrared ray cut filter is separately provided on the surface of the hermetically sealed package, satisfactory improvement cannot be made with respect to difficulty in manufacture and/or assembling accuracy.
In addition, an image sensor system in which infrared ray cut filter and infrared ray transmission filter are formed on the solid state image sensing device is disclosed in the Japanese Patent Application Laid-Open Publication No. 60-53922(1985). However, since filters of materials different from material of color filters are used as respective filters, there still remained problems in respect of the accuracy and the cost.